Photographic processes for obtaining direct positive images without employing a reversal processing step or a negative film are well known.
Taking practical utility into consideration, conventional techniques for obtaining a positive image from a direct positive silver halide photographic light-sensitive material, exclusive of certain special methods, are divided chiefly into the following two types.
One type employs a previously fogged silver halide emulsion whose fog centers (latent image) in exposed areas are destroyed making use of the solarization or Herschell effect to obtain a direct positive image, after development.
The other type uses an internal latent image type silver halide emulsion not having been fogged, which is imagewise exposed to light and then subjected to surface development either after fogging treatment or during fogging treatment to obtain a direct positive image. The internal latent image type silver halide emulsion used herein is an emulsion in which silver halide grains have sensitivity specks predominantly in the interior thereof and form a latent image predominantly in the interior upon exposure to light.
The various methods belonging to the latter type generally enjoy higher sensitivity and are suitable for uses requiring high sensitivity as compared with the various methods of the former type. The method of the present invention belongs to the latter type.
Various techniques of this type have been known, such as those disclosed, for example, in U.S. Pats. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 2,761,276, and 3,796,577, and British Pats. 1,151,363, 1,150,553, and 1,011,062. According to these conventional techniques, photographic materials providing a direct positive image with relatively high sensitivity can be produced.
For the details of the direct positive image formation mechanism, reference can be made to, for example, T.H. James, The Theory of the Photographic Process, 4th Ed., Ch. 7, pp. 182 to 193 and U.S. Pat. No. 3,761,276.
It is believed that a direct positive image is formed through the following mechanism: first, imagewise exposure results in the formation of an internal latent image in the inside of silver halide grains, which leads to the formation of fog centers selectively on the surface of the unexposed silver halide grains by surface desensitization based on the internal latent image, and subsequent conventional surface development processing results in formation of a photographic image (a direct positive image) on the unexposed area.
Selective formation of fog centers can be generally effected by a "light fogging method" in which the entire surface of a light-sensitive layer is secondarily exposed to light as described, for example, in British Pat. No. 1,151,363 or a "chemical fogging method" using a nucleating agent as described, for example, in Research Disclosure, Vol. 151, No. 15162 (November, 1976), pp. 76 to 78.
In the formation of a direct positive color image, the internal latent image type silver halide light-sensitive material is subjected to surface color development processing either after or simultaneously with fogging treatment and is then subjected to bleaching and fixing (or bleach-fixing). After the bleaching and fixing processing, the material is usually washed with water and/or subjected to stabilizing processing.
According to direct positive image formation by the above-described light fogging method or chemical fogging method, the rate of development is lower, requiring a longer processing time as compared with general negative type photographic materials. Hence, the pH and/or temperature of the developing solution used in these methods may be increased to thereby reduce the development time. However, the use of a developing solution having a higher pH value generally causes an increase in the minimum image density of the resulting direct positive image. Further, the developing agent is more susceptible to deterioration due to air oxidation under a high pH, and the pH is apt to decrease over time due to absorption of carbon dioxide in the air, so that development activity becomes seriously reduced.
In order to solve these problems, compounds which exhibit a nucleating function under a developing solution pH of 12 or lower have been proposed as described, for example, in JP-A-52-69613 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and U.S. Pats. 3,615,615 and 3,850,638. However, these compounds also cause an undesired increase in the minimum image density. In addition, the maximum image density obtained is sufficiently low.
On the other hand, certain merocyanine series sensitizing dyes have been employed in a latent image type silver halide emulsion in order to increase the maximum image density as described in U.S. Pat. No. 4,306,016. However, the degree of increase is still unsatisfactory.
Moreover, the increase in the development time in the development processing of internal latent image type direct positive color photographic light-sensitive materials is also problematic in that the toe portion of the characteristic curve of the image obtained tends to be flat.